The present invention generally relates to a frequency and/or phase characteristic control circuit. More particularly, the invention relates to a frequency and/or phase characteristic control circuit conducive to being formed in an IC (integrated circuit) for the phase compensation of servo circuits in applications such as optical disc recording and reproducing apparatus.
In Japanese Laid-open Utility Model No. 63-6516 there is disclosed a phase compensation circuit that includes a switching circuit for selecting a time constant. FIG. 1 of the present application is a reproduction of such circuit and illustrates a circuit diagram of a typical frequency and/or phase characteristic control circuit wherein a signal is fed from a photo detector 1 to a focus error amplifier 3 via current/voltage converters 2. The signal is then applied to an in-phase input terminal of an operational amplifier 4. From there, the signal is sent to a voltage/current converter 5 which in turn drives a focus coil 6.
As illustrated, coupled to another input of the operative amplifier 4 are two selectable time constant establishing branches 4a and 4b. Branch 4a comprises serially coupled reactor R.sub.1 and capacitor C.sub.1. Branch 4b comprises serially coupled resistor R.sub.2 and capacitor C.sub.2. These time constant branches are selected in accordance with the application of a control signal at a control terminal C.sub.1. Inverters IV.sub.3 and IV.sub.1 and IV.sub.2 are operatively disposed between the control terminal CT and the branches 4a and 4b, respectively, to provide for such selection.
One disadvantage of this prior art phase compensation circuit is its inability to be readily formed in an IC. This is because the time constant capacitor C.sub.1 or C.sub.2 needs to have a capacity of about 0.1 .mu.F when the responsive frequency is to be switched between 1 KHz (e.g., for a compact disc player) and 2 KHz (e.g., for a laser disc player).